Infrared radiation (IR) is well known in heating applications and has been implemented in foodservice for functions like toasting and broiling. Infrared radiation comprises energy waves that are generated by an emitter and travel at the speed of light, in straight lines until intercepted by an object. When an object is encountered the energy waves can be absorbed, reflected, or transmitted through the object where they continue to travel until encountering another object. When the infrared energy waves are absorbed; the energy converts to heat within the object that has absorbed the energy resulting in heating of the object. This process is known as radiant heating.
Infrared radiation energy waves are part of the electromagnetic spectrum and are classified by their wavelength and frequency. Infrared radiation energy waves are identified as a segment of the electromagnetic wave spectrum that lies between visible light and microwaves. IR displays characteristics of visible light at the shortest wavelength extreme and microwaves at the longest wavelength extreme. The infrared electromagnetic spectrum ranges approximately between 0.7-microns and 400-microns. Segments of the span are considered to be near IR (0.7 to 1.3 microns), mid IR (1.3 to 3.0 microns) and far IR (3.0 to 400 microns). The infrared wavelengths used most for thermal heat transfer are typically in the range of 3-30 microns. Infrared wave emitters used for thermal applications in food processing usually fall between 1.0 and 9.0 microns.
Infrared radiation heating is ideal for non-contact heating. IR heating has the ability to heat an object without the energy emitter contacting the object, unlike convection heating or conductive heating which both convey heat by a medium in contact with the object being heated. Radiated infrared energy does not heat the space through which the IR travels so long as any material in that space does not absorb the IR.
Steam is another well-known heating method. Steam has been implemented in foodservice for steam cooking, food/water heating, and food holding applications. Steam is a powerful heating medium in that steam fills voids in confined spaces. Steam is also powerful for heating in that steam stores latent heat that is released on contact with a colder object resulting in the object being heated. The latent heat potential is obvious if one has ever had their skin come in contact with 212° F. steam, a burn and blistering can be immediate; however putting one's hand in a 212° F.+ oven does not result in immediate burning.
When applying infrared heat or steam to perform heating certain problems can occur. For example an object will receive non-uniform heating based on the pattern of the IR waves impinging upon the object. If the emitter is in a figure eight, the object to be heated will receive a pattern of infrared waves in a figure eight. This figure eight pattern will be absorbed as energy and will create heat in a figure eight pattern in the object to be heated. The balance of the object outside the figure eight pattern will have to be heated by conduction through the object from the figure eight pattern. This characteristic of IR can result in overheating and burning in the area where the energy is directly absorbed and under-heating in other areas where in heat is not directly absorbed.
Steam condenses when the steam touches any object colder than the temperature of the steam and leaves a water film at the location of condensation. It is difficult to direct steam just to the object to be heated, since the steam fills all voids and touches all surfaces very quickly. This process of heating all surfaces, and not just the surfaces of the product intended to be heated, requires more latent heat and thus more steam to be supplied to bring all surfaces to the desired temperature. Another problem is that the water film left by condensing steam can serve as an insulation barrier to the object to be heated; this requires new steam to first heat the water layer and then the object to be heated, making steam heating less efficient.
An important consideration in using IR for heating is the output wavelength generated by the emitter since different wavelengths have different characteristics. Furthermore, the IR energy waves that reach an object to be heated display different characteristics depending on the physical nature of the object. When creating or selecting an emitter for heating applications it is important to select or create an emitter that is “tuned” to an optimum wavelength that is most appropriate for the application. An emitter tuned to the optimum wavelength will provide a higher percentage of the optimum wavelength and will heat most efficiently for the application. Wavelengths that are not absorbed by the object do not take part in heating as they reflect away from the object or are transmitted through the object. These non-absorbed energy wavelengths travel until they encounter an object that is more tuned to absorb their particular wavelength.
Absorption and emissivity are two properties of an object that directly follow each other. These properties determine how well the object will absorb infrared energy waves. Objects are rated in emissivity values on the basis of a blackbody rating which is valued at 1 for a perfect black body. A perfect black body would absorb 100-percent of the radiant energy that strikes it. A perfect black body does not exist.
Absorption of radiation is a selective phenomenon depending greatly on the incident wavelength. Most of the emitters used in food heating range from peak values of 1.0-9.0 microns. Planck's curves, a known research tool, show that a film of water thicker than 0.05 mm will absorb much of the infrared radiation longer than 2.5-microns. Water is rated very high in absorption with an emissivity value of 0.93 for wavelengths greater than 2.5-microns. The water vapor will absorb little of the infrared radiation shorter than 2.5-microns. When IR radiant energy shorter than 2.5-microns encounter water film or vapor most of the short wavelength infrared (1.0-2.5 microns) is transmitted though the water or vapor without absorption. The short wavelengths are closer to light on the electromagnetic spectrum, and like light through a window, the energy travels through the vapor. The longer wavelengths greater than 2.5-microns, however, are further from the visible light wavelength and are absorbed by the water molecules creating an increase in temperature in the water molecules.
The differences in the spectral absorption/transmission of water layers and water vapor can be observed in FIGS. 1a and 1b. Details of the associated mechanisms are described for FIG. 1a in “Radiant Heating with Infrared” by Watlow Electric Manufacturing Company, and for FIG. 1b in “Infrared Absorption by Water Clusters” by Hugh R. Carion, the entire disclosure of both which are herein incorporated by reference.
Rapid and high quality preparation of food is required in today's food industry. The present patent application puts forth a novel apparatus that combines infrared radiant energy and steam in unique ways to provide a combination of direct and indirect heating of food that achieves improved thermal treatment of food resulting in faster cooking times and more uniform heating of the food.